Substrates such as self-supporting film structures of organic thermoplastic polymeric material are often coated on one or both planar surfaces with a heat-curable composition. Heretofore, such coatings have been cured by passing the coated film structure through a heated chamber, and such processes usually require festooning the coated film structure in the heated chamber in order to provide adequate hold-up time in a chamber of reasonable size whereby to achieve economical operation. Various attempts have been made to provide better and improved processes for curing heat-curable coatings on substrate materials. An improved process is disclosed in U.S. Pat. No. 2,426,775 which describes passing a web consisting of a single sheet of fabric coated on one side with a heat-curable coating or consisting of two sheets of fabric and an interlayer therebetween of un-cured rubber composition into a heated chamber, and rapidly heating the web to a predetermined temperature followed by winding the heated web into roll form, and thereafter storing the rolled web at constant elevated temperature until the curing reaction is completed afterwhich the web is unrolled and cooled as by passing the web over cold idler rolls. A separator sheet such as "Holland" cloth is wound between the adjacent layers of the rolls to prevent the layers from sticking together. Also, the use of an interleaf between layers of a roll of organic thermoplastic film material is a well known expedient for preventing the layers of the roll from sticking together. For instance, U.S. Pat. Nos. 2,303,826; 2,303,828 and 3,524,778 each discloses the use of an interleaf for preventing the layers of a roll of polyvinyl acetal resin from sticking together. The interleaf may be of diverse materials such as a cellulose plastic, e.g., cellophane, polyethylene or a fibrous material, e.g., kraft paper, coated with cellulose acetate. The use of roll curing methods for curing heat-curable coatings on film structures having a high modulus, e.g., polyester films, with the use of interleaves having a smooth surface or without any interleaf is characterized by several drawbacks or major disadvantages, namely, flat spots, splitting, blocking and wrinkling of the film structure. Flat spots are surface irregularities on a film structure that contribute to poor optical appearance and are caused by air pockets entrapped in a roll during the winding sequence or pockets of entrapped gases that are evolved from a coating during the curing thereof. These air or gas pockets result in the non-uniform treatment of the rolled film structure when heated. Splitting of a roll film structure is caused by the accumulation of stress in the roll layers due to the winding tension employed and ordinarily occurs when the roll of film is heated. Blocking is a surface phenomenon characterized by intimate contact between adjacent layers of a roll of such an extensive nature as to render parting of the layers extremely difficult; blocking is analogous to sticking in that the latter is an extreme form of the former. Wrinkling is caused by non-uniform stresses that occur in the layers of a roll upon heating. Accordingly, the principal object of the present invention is to provide a method for curing a heat-curable coating on one or both surfaces of a flexible substrate that overcomes the foregoing drawbacks.